Plant stomata are composed of two guard cells, and their opening has many regulatory mechanisms, one of which is shown in the figure (ABA stands for abscisic acid).

A, when the concentration of SO2 is too high, it will inhibit the activity of CO2 immobilized enzyme, and then inhibit the dark reaction of photosynthesis, which will reduce the photosynthetic rate, so A is correct;

B, ABA can adjust the opening and closing of stomata, so it can help plants avoid SO2 damage to a certain extent, so B is correct;

C, as can be seen from the figure, when the concentration of SO2 is too high, the activity of CO2 immobilized enzyme will be inhibited, and the content of C3 compounds in plants will be reduced, so c is wrong;

D, CO2 in and out of the cell is free diffusion, so D is correct.

So choose C.